The present invention relates to a communication environment measurement method for a mobile station and a mobile station, and more particularly to a communication environment measurement method for a mobile station in a communication system where data is transmitted from one base station to the mobile station, and the mobile station.
A W-CDMA (UMTS) mobile communication system is a radio communication system where a line is shared by a plurality of users, and comprises a core network 1, radio base station controllers (RNC: Radio Network Controller) 2 and 3, multiplexers/demultiplexers 4 and 5, radio base stations (Node B) 61-65 and mobile station (UE: User Equipment) 7, as FIG. 15 shows.
The core network 1 is a network for routing in the mobile communication system, and the core network can be constructed by an ATM switching network, a packet switching network or a router network, for example. The core network 1 is also connected with a public network (PSTN), so that the mobile station 7 can communication with fixed telephones.
The radio base station controllers (RNC) 2 and 3 are positioned as the host of the radio base stations 61-65, and have a function to control these radio base stations 61-65 (e.g. management of radio resources to be used). The radio base station controllers 2 and 3 also have a handover control function, which is a function for receiving signals sent by one mobile station 7 from a plurality of radio base stations at hand over, selecting data signal having the best quality, and sending it to the core network 1 side.
The multiplexers/demultiplexers 4 and 5 are installed between the RNC and a radio base station, and perform control to demultiplex the signals addressed to each radio base station received from the RNCs 2 and 3, and outputs them to each radio station as well as to multiplex signals from each radio station, and transfers them to each RNC.
The radio resources of the radio base stations 61-63 are managed by the RNC 2, and the radio resources of the radio base stations 64 and 65 are managed by the RNC 3, to perform radio communication with the mobile station 7. The mobile station 7, which exists in one of the radio areas of the radio base stations 61-65, establishes the radio line with one of the radio base stations 61-65, and communicates with another communication device via the core network 1.
The interface between the core network 1 and the RNCs 2 and 3 is called the Iu interface, the interface between the RNCs 2 and 3 is called the Iur interface, and the interface between the RNCs 2 and 3 and each radio base station 61-65 is called the Iub interface, the interface between the radio base stations 61-65 and the mobile station 7 is called the Uu interface, and the network composed of devices 2-6 in particular is called a radio access network (RAN). The lines between the core network 1 and the RNCs 2 and 3 are shared by Iu and Iur interfaces, and the lines between the RNCs 2 and 3 and the multiplexers/demultiplexers 4 and 5 are shared by Iub interfaces for a plurality of radio base stations.
The above is a description on a general mobile communication system, but now a technology to allow high-speed downstream data transmission, such as HSDPA (High-Speed Downlink Packet Access), is becoming incorporated into mobile communication systems (see 3G TS 25.212 (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and Channel Coding (FDD)); and 3G TS 25.214 (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Physical Layer Procedure (FDD)).
HSDPA
HSDPA is a method for switching the transmission rate according to the radio environment between a radio base station and a mobile station, and switches the data size per one transport block depending on the radio environment, or adaptively switches the encoding modulation method. In the case of adaptive modulation and coding (AMC), the QPSK modulation scheme and the 16QAM scheme are adaptively switched, for example.
The HSDPA uses H-ARQ (Hybrid Automatic Repeat reQuest). In H-ARQ, if the mobile station detects an error in the receive data from the radio base station, a retransmission request (NACK signal) is sent to the radio base station. The radio base station that received this retransmission request retransmits the data, so the mobile station performs the error correction decoding using the already received data and the retransmitted receive data. In this way, in the case of H-ARQ, the already received data is effectively used even if an error occurs, so the gain of the error correction decoding increases and as a result the retransmission count can be suppressed to be low. If an ACK signal is received from a mobile station, data transmission is a success and retransmission is unnecessary, so the next data is transmitted.
The main radio channels to be used for HSDPA are, as FIG. 16 shows, (1) HS-SCCH (High Speed-Shared Control Channel), (2) HS-PDSCH (High Speed-Physical Downlink Shared Channel), and (3) HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Both HS-SCCH and HS-PDSCH are shared channels in the downstream direction (downlink from the radio base station to a mobile station), and HS-SCCH is a control channel for transmitting various parameters on the data to be transmitted via HS-PDSCH. In other words, this is a channel for notifying that the data is transmitted via HS-PDSCH. The various parameters include, for example, the destination information of the mobile station to which the data is transmitted, the transmission bit rate information, the modulation scheme information on the modulation scheme with which data is transmitted via the HS-PDSCH, the number of allocated spreading codes (code count), and the pattern of the rate matching to be performed on the transmission data.
The HS-DPCCH, on the other hand, is a dedicated control channel in the upstream direction (uplink from a mobile station to a radio base station), and is used to transmit the respective receive result (ACK signal, NACK signal), depending on the presence of an error in the data received via the HS-PDSCH, to the radio base station. In other words, this is a channel to be used to transmit the receive result of the data received via the HS-PDSCH. If the mobile station fails in received data (e.g. receive data has a CRC error), the NACK signal is transmitted from the mobile station, so the radio base station executes retransmission control.
The HS-DPCCH is also used when the mobile station, which measures the receive quality such as SIR of the signal received from the radio base station, transmits this receive quality to the radio base station as CQI (Channel Quality Indicator). In other words, CQI is the information for the mobile station to report the receive environment to the base station and, for example, CQI value is 1-30, where CQI value is determined so that the block error rate BLER, does not exceed 0.1 and reported to the base station.
For example, the mobile station holds a CQI table, and determines a CQI value corresponding to the receive quality (SIR) from this CQI table, and transmits this value to the radio base station via the HS-DPCCH.
The radio base station judges whether the radio environment in the downstream direction is good or not by the received CQI, and if good, the radio base station switches the modulation scheme to one that can transmit data faster, and if not good, it switches the modulation scheme to one that transmits data slower (that is performs adaptive modulation). Actually the base station has a CQI table which defines formats with different transmission speeds according to CQI=1-30, and determines the parameters (e.g. transmission speed, modulation method, multiplexed code count) according to the CQI value obtained from this CQI table, and notifies these parameters to the mobile station by HS-SCCH, and also transmits the data to the mobile station by HS-PDSCH based on these parameters.
Channel Structure
FIG. 17 is a diagram depicting the timing of the channels in the HSDPA system. In W-CDMA, which uses code division multiple access, the codes separate each channel. The CPICH (Common Pilot Channel) and the SCH (Synchronization Channel) are shared channels in the downstream direction. The CPICH is a channel used at a mobile station for channel estimation and cell search, and is a channel for transmitting the so called pilot signals. SCH is further divided into P-SCH (Primary SCH) and S-SCH (Secondary SCH), and is a channel which is transmitted in bursts by the first 256 chips of each slot. This SCH is received by a mobile station which performs three-level cell search, and is used for establishing slot synchronization and frame synchronization and for identifying the base station code (scramble code). SCH is 1/10 the length of one slot, but is shown to be a little wider than this in FIG. 17. The remaining 9/10 is P-CCPCH (Primary-Common Control Physical Channel).
Now the timing relationship of the channel will be described. In each channel, 15 slots constitute one frame (10 ms), and one frame has a length equivalent to 2560 chip lengths. As described above, CPICH is used as a reference for other channels, so the beginning of the frame of the SCH and HS-SCCH match the beginning of the frame of CPICH. The beginning of the frame of the HS-PDSCH is two slots delayed from the HS-SCCH, but this is because the mobile station receives the modulation scheme information via HS-SCCH, and then enables the demodulation of HS-PDSCH by a demodulation scheme according to this modulation scheme. In HS-SCCH and HS-PDSCH, three slots constitute one sub-frame.
The HS-DPCCH is a channel in an upstream direction, and the first slot thereof is used to transmit an ACK/NACK signal to show the receive result of HS-PDSCH from the mobile station to the radio base station when about 7.5 slots elapse after the receipt of HS-PDSCH. The second and third slots are used to feedback and regularly transmit the CQI information for adaptive modulation control to the base station. The CQI information to be transmitted is calculated based on the receive environment (SIR measurement result of CPICH) measured during the period of four slots before to one slot before in the CQI transmission.
Handover
The mobile station 7 is communicating data via the HS-PDSCH with the base station 61 of the serving cell (see (A) of FIG. 18). At this time, handover status occurs if the mobile station 7 approaches an adjacent cell (non-serving cell) by moving (see (B) of FIG. 18). And when the quality of signals received from the base station 62 of the non-serving cell, such as SIR (Signal to Interference Ratio), becomes better than the SIR of the signals received from the base station 61 of the serving cell, the RNC switches the communication base station from the base station 61 to the base station 62 (see (C) of FIG. 18), and transmits data from the base station 62 to the mobile station 7 via HS-PDSCH.
The downstream signal from each cell has a different scrambling code, so each signal is demultiplexed by de-spreading using the respective scrambling code at a mobile station. The receive signal includes a common pilot signal CPICH, so the mobile station de-spreads the receive signal by the scrambling code and demultiplexies CPICH signal base station by base station. Thereafter the CPICH signal is multiplied by the channelization code for de-spreading, and by this the average power of CPICH signal and the variance value thereof are derived, and SIR is determined for each cell using the power of CPICH signal. And the SIR of each cell is compared one another, and the cell having the highest SIR is notified to the base station as a candidate of handover destination.
FIG. 19 shows the sequence of handover, and in HSDPA, handover is performed as a hard handover.
When the mobile station 7 is communicating with the base station 61 in the serving cell (step S1), and when SIR, which is the receive quality from the base station 62 of the non-serving cell, becomes good, the mobile station 7 notifies the SIR of the signal received from each base station 61 and 62 to the RNC 2 via the higher logical channel DCCH (step S2). When the SIR report which is channel switching request is received, the RNC 2 instructs the base station 62 to start up the communication channel (HS-PDSCH) allocated to the communication between the mobile station 7 and the base station 62 Of the non-serving cell (handover request, step S3). When the instruction to start up the communication channel is received, the base station 62 responds with a confirmation (step S4).
Then the RNC 2 notifies the communication channel (HS-PDSCH) of the handover destination to the mobile station 7 via the base station 61 during communication (step S5). When the information on the communication channel of the handover destination is received, the mobile station 7 immediately switches the channel according to the communication channel, and enables communication with the base station 62, and hereafter transmits/receives synchronization burst signals and communication burst signals to establish frame synchronization and to adjust time alignment with the destination base station 62. And when normal communication becomes possible, the base station 62 of the serving cell reports the channel start-up completion to the RNC 2 (handover: step S6). When the channel start-up completion signal is received, the RNC 2 sends an instruction to release the channel to the base station 61, and ends handover (step S7). Hereafter the mobile station 7 communicates data with the base station 62 via HS-PDSCH. At this time, HS-SCCH is also switched and a reception of data via HS-PDSCH is attempted when data which is transmitted from the base station 62 via HS-SCCH is received.
Problems of Conventional Handover
To perform handover control, a mobile station measures the SIR quality of the receive signals from the serving cell, which is currently transmitting the HS-PDSCH signals, and the SIR quality of the receive signals from other non-serving cells. To measure these SIRs, power of CPICH signal, which is transmitted from each cell, is used. The ratio of the CPICH power to the total receive power from the serving cell, where HS-PDSCH signals are transmitted to the mobile station, is smaller than the ratio to the total receive power from the non-serving cells where HS-PDSCH signals are not transmitted to the mobile station.
The mobile station, in which an analog circuit is used for the receiver, generates fixed noise components by NF (Noise Figure) of the receiver, waveform distortion due to a filter and local phase noise. The CPICH signal of each cell is influenced by internal noise in the receive step, and in the signals from the base station transmitting data via HS-PDSCH, the SN ratio is small since the ratio of CPICH power to the total receive power is small, and as a result SIR is small.
As described above, SIR of the serving cells may decrease during handover by the amount of the influence of the HS-PDSCH signal. On the other hand, when another cell is not transmitting data via HS-PDSCH or when the power thereof is small, the deterioration of SIR could be small since the interference of HS-PDSCH is minimal. As a result, handover to the other cell which is an non-serving cell, is likely to occur easily. And if handover occurs here, the transmission of the signals of HS-PDSCH shifts to the other cell as a new serving cell. As a result, the data is transmitted from the new serving cell to the mobile station via HS-PDSCH instead, and interference increases and the SIR value of the new serving cell tends to drop. In the old serving cell, on the other hand, the data is not transmitted to the mobile station via HS-PDSCH after handover has completed, therefore interference decreases, SIR increases and SIR of the old serving cell may be higher than SIR of the new serving cell. HS-PDSCH is a hard handover, so every time a handover occurs the communication is interrupted and the throughput drops.
The reason why the SIR of signals from the base station, which transmits data through HS-PDSCH, decreases will be described. The power of the signals received by the antenna is composed of CPICH power, HS-PDSCH power, other channel power and external noise power, as shown in FIG. 20. The total receive power is the total of these powers, and as FIG. 21 shows, internal noise according to the value of this total receive power is generated. The total receive power is given by the following expression.
                              Total          ⁢                                          ⁢          receive          ⁢                                          ⁢                      power            ⁢                                                  [            dBm            ]                          =                  10          ⁢                                          ⁢                      log            10                    ⁢                      {                                          CPICH                ⁢                                                                  ⁢                                  power                  ⁢                                                                          [                  mW                  ]                                            +                              HS                ⁢                                  -                                ⁢                PDSCH                ⁢                                                                  ⁢                                  power                  ⁢                                                                          [                  mW                  ]                                            +                              other                ⁢                                                                  ⁢                channel                ⁢                                                                  ⁢                                  power                  ⁢                                                                          [                  mW                  ]                                            +                              external                ⁢                                                                  ⁢                noise                ⁢                                                                  ⁢                                  power                  ⁢                                                                          [                  mW                  ]                                                      }                                              (        1        )            The internal noise power is a value corresponding to the total receive power, and is given by the following expression,
Internal noise power [mW]=total receive power [mW]/η, and this is converted to the following expression using by logarithm,Internal noise power [dBm]=total receive power [dBm]−internal noise power ratio [dB]  (2)In addition, the total noise power can be given byTotal noise power [dBm]=10 log10{external noise power[mW]+internal noise power [mW]}  (3)and SIR, which is calculated from CPICH, is in proportion to the ratio of the CPICH power and the total noise power, as shown in FIG. 22, so SIR can be calculated by the following formula.SIR [dB]=CPICH power [dBm]−total noise power [dBm]+CPICH spreading gain [dB]  (4)Here it is assumed that the signals of HS-PDSCH have stopped, as shown in FIG. 23. When HS-PDSCH power stops, the total receive power decreases, as shown in FIG. 24, and according to this, the internal noise power and the total noise power decrease. Since the CPICH power does not change depending on whether HS-PDSCH power stops or not, as a result SIR increases, as shown in FIG. 25.
As described above, SIR is measured to be low when data is being received via HS-PDSCH, and SIR is measured to be high when data is not being received via HS-PDSCH. In other words, the SIR of the signal from a base station which is transmitting data via HS-PDSCH decreases, and the SIR of the signal from a base station which is not transmitting data via HS-PDSCH increases.
In this way, if a mobile station moves and a handover occurs during HS service (during status of waiting for receipt of data via HS-PDSCH), SIR tends to drop in a new serving cell and SIR tends to increase in a old serving cell, and this influence is particularly strong when high-speed transmission is performed at high power.
Because of this, the SIR measurement conditions differ between the serving cell and non-serving cell, and handover tends to occur sooner rather than at the correct timing.
In the case of HS-PDSCH, which uses a hard handover, the power of the HS-PDSCH shifts to the next base station after handover completes, so the SIR of the next base station is measured low while the SIR of the previous base station is measured high, which causes handover again.